Power distribution systems are well known. In a typical arrangement of a marine power distribution and propulsion system shown in FIG. 1 a plurality of ac generators G1-G4 provide ac power to a busbar or switchboard 2 which carries a fixed-frequency distribution voltage such as 690 V, 60 Hz. Each generator G1-G4 is coupled to a prime mover such as a diesel engine D1-D4.
Electric propulsion motors M1-M4 are connected to the busbar 2 by means of interposing power converters 4. The propulsion motors M1-M4 can be of any suitable type and construction and can optionally be configured to drive a propeller shaft or other propulsion system such as a thruster.
In some arrangements, the interposing power converters 4 can be active front end (AFE) power converters. An AFE power converter typically includes a first active rectifier/inverter 6 having ac terminals connected to the busbar 2 and a second active rectifier/inverter 8 having ac terminals connected to the propulsion motor M1-M4. The dc terminals of the first and second active rectifier/inverters 6, 8 are connected together by a dc link 10. A harmonic filter 12 is normally connected to the ac terminals of the first active rectifier/inverter 6, i.e. on the network-side, to ensure harmonic voltages and currents are eliminated. The AFE power converters might, for example, be implemented as MV3000 converters supplied by GE Energy Power Conversion UK Ltd of Boughton Road, Rugby, United Kingdom.
In normal operation, the first active rectifier/inverter 6 will operate as an active rectifier to supply power to the dc link 10 and the second active rectifier/inverter 8 will operate as an inverter to supply power to the associated propulsion motor M1-M4. However, reverse operation can be possible in certain situations such as regenerative braking for the propulsion motors M1-M4. During regenerative braking the propulsion motors M1-M4 generate power which can be fed back to the busbar 2. In this situation the second active/rectifier 8 will operate as an active rectifier to supply the generated power to the dc link 10 and the first active rectifier/inverter 6 will operate as an inverter to supply power to the busbar 2.
Each active rectifier/inverter 6, 8 will typically have a suitable topology with semiconductor power switching devices fully controlled and regulated using a pulse width modulation (PWM) strategy.
The busbar 2 can be equipped with protective switchgear with circuit breakers and associated controls. The busbar 2 will typically be divided into a pair of busbar sections 2a, 2b (e.g. port and starboard) that are interconnected by a tie 14. The actual arrangement of the power distribution system will typically depend on redundancy, which is particularly important for marine vessels.
The generators G1=G4 and power converters 4 can be connected to the busbar 2 by circuit breakers 16, 18 and associated controls or other switching means.
A conventional power distribution system can have any suitable number and type of generators and any suitable busbar configuration depending on the power generation and distribution requirements.
There are times when it may be for the marine vessel to be able to operate with the main generators G1-G4 off-line. This might include times when the marine vessel is in port or when there is a need to try and minimise the emission of environmentally harmful exhaust gases such as nitrogen oxides (NOx), carbon dioxide (CO2) and other pollutants that are related to the combustion process of the diesel engines D1-D4. There are also times when the generators G1-G4 might be off-line because of a critical fault or for safety reasons. It will therefore be readily appreciated that there is a need for an alternative source of power when the power that would normally be provided by the main generators G1-G4 is unavailable.